Wafer processing method

ABSTRACT

A wafer is processed by transferring a wafer to a holding surface of a chuck table by using a suction pad. The front side of the wafer is held through a protective tape on the holding surface under suction. The suction pad is then removed from the back side of the wafer and the back side of the wafer is ground, thereby thinning the wafer and also dividing the wafer into individual device chips. The wafer is mounted on the holding surface while held by the suction pad. The wafer is sandwiched between the suction pad and the holding surface when the suction force is removed. A suction force is applied to the holding surface to thereby hold the front side of the wafer through the protective tape on the holding surface, and the suction pad is then removed from the back side of the wafer.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer into individual device chips by applying a laser beam to thewafer in the condition where the focal point of the laser beam is setinside the wafer to thereby form a modified layer inside the wafer.

Description of the Related Art

A plurality of devices such as integrated circuits (ICs) and large scaleintegrations (LSIs) are formed on the front side of a wafer so as to beseparated by a plurality of crossing division lines. The wafer thushaving the plural devices is divided along the division lines intoindividual device chips by using a dicing apparatus or a laserprocessing apparatus, for example. The device chips divided from thewafer are used in electrical equipment such as mobile phones andpersonal computers. Further, there has been proposed a techniqueincluding the steps of applying a laser beam having a transmissionwavelength to the wafer to the back side of the wafer along eachdivision line in the condition where the focal point of the laser beamis set inside the wafer to thereby form a modified layer inside thewafer along each division line, and next grinding the back side of thewafer to thereby thin the wafer and also divide the wafer into theindividual device chips (see Japanese Patent Laid-open No. 2014-007257,for example).

According to the technique disclosed in Japanese Patent Laid-open No.2014-007257, the thickness of each device chip can be reduced and thedie strength thereof can also be improved as compared with the case ofusing a conventional dicing apparatus to form a division start pointalong each division line.

SUMMARY OF THE INVENTION

The present inventors have found that in performing the steps ofapplying a laser beam to the back side of a wafer to form a modifiedlayer inside the wafer and next grinding the back side of the wafer tothereby thin the wafer and also divide the wafer into the individualdevice chips, there is a case that the wafer may be broken at a positiondifferent from each division line where the modified layer is formed.Such a breaking phenomenon causes a problem such that the devices formedon the front side of the wafer may be partially damaged to remarkablyreduce the production efficiency of each device chip.

Under these circumstances, the present inventors have closely studiedthe cause of the above breaking phenomenon to obtain the followingfindings. After performing a modified layer forming step of applying alaser beam to the back side of a wafer in a laser processing apparatusto form a modified layer inside the wafer, the wafer is unloaded from achuck table included in the laser processing apparatus and thentransferred to a chuck table included in a grinding apparatus by usingtransfer means having a suction pad. In the grinding apparatus, the backside of the wafer held on the chuck table is ground to thin the waferand divide the wafer into the individual device chips. In transferringthe wafer from the chuck table of the laser processing apparatus to thechuck table of the grinding apparatus, the wafer is held by the suctionpad under suction. At this time, a nonuniform internal stress isgenerated inside the wafer by a suction force applied to the suctionpad. When this internal stress is not sufficiently relieved and thewafer is then held on the chuck table of the grinding apparatus undersuction, the internal stress due to the suction holding by the suctionpad is partially left inside the wafer. In the next grinding step, agrinding pressure is applied to the wafer to grind the back side of thewafer and thereby thin the wafer. Due to the application of the grindingpressure, the residual internal stress causes the breaking phenomenon atan unintentional position.

It is therefore an object of the present invention to provide a waferprocessing method which can prevent the breaking phenomenon at anunintentional position in a wafer in performing the step of grinding theback side of the wafer to thin the wafer and also divide the wafer intothe individual device chips.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer into a plurality ofindividual device chips along a plurality of crossing division linesformed on the front side of the wafer, the front side of the wafer beingpartitioned by the division lines to define a plurality of separateregions where a plurality of devices are formed, the individual devicechips corresponding to the respective devices. The wafer processingmethod includes a protective tape attaching step of attaching aprotective tape to the front side of the wafer; a holding step ofholding the protective tape attached to the front side of the wafer on aholding surface of a first chuck table under suction; a modified layerforming step of applying a laser beam having a transmission wavelengthto the wafer to the back side of the wafer along each division line inthe condition where the focal point of the laser beam is set inside thewafer after performing the holding step, thereby forming a modifiedlayer inside the wafer along each division line; an unloading step ofholding the back side of the wafer held on the first chuck table byusing a suction pad of transfer means after performing the modifiedlayer forming step, and next moving the suction pad to thereby unloadthe wafer from the first chuck table; a transfer step of transferringthe wafer to a holding surface of a second chuck table by operating thetransfer means after performing the unloading step, next holding theprotective tape attached to the front side of the wafer on the holdingsurface of the second chuck table under suction, and next removing thesuction pad from the back side of the wafer; and a grinding step ofgrinding the back side of the wafer held on the second chuck table undersuction, thereby thinning the wafer and also dividing the wafer into theindividual device chips. The transfer step includes a mounting step ofmounting the wafer held by the suction pad to the holding surface of thesecond chuck table; a sandwiching step of removing a suction forceapplied to the suction pad after performing the mounting step, and thensandwiching the wafer between the suction pad and the holding surface ofthe second chuck table; and a suction holding step of applying a suctionforce to the holding surface of the second chuck table after performingthe sandwiching step to thereby hold the protective tape attached to thefront side of the wafer on the holding surface of the second chuck tableunder suction, and next removing the suction pad from the back side ofthe wafer.

According to the present invention, in transferring the wafer from thesuction pad to the second chuck table, the suction force applied to thesuction pad holding the wafer under suction is removed to therebyrelieve an internal stress in the wafer. Accordingly, the internalstress due to the suction holding by the suction pad is not left in thewafer and the wafer is next held on the second chuck table undersuction. As a result, even when the back side of the wafer held on thesecond chuck table is ground to reduce the thickness of the wafer andalso divide the wafer into the individual device chips in the grindingstep, there is no possibility that the devices may be partially damaged.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and an appended claim with reference to theattached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a protective tape attaching step ofattaching a protective tape to a silicon wafer;

FIGS. 2A and 2B are perspective views showing a holding step of holdingthe silicon wafer on a first chuck table of a laser processingapparatus;

FIG. 3A is a perspective view showing a modified layer forming step offorming a modified layer inside the silicon wafer;

FIG. 3B is an enlarged sectional view of the silicon wafer shown in FIG.3A;

FIG. 4 is a perspective view schematically showing an unloading step ofunloading the silicon wafer from the first chuck table;

FIGS. 5A to 5C are side views for specifically illustrating theunloading step shown in FIG. 4;

FIGS. 6A and 6B are perspective views schematically showing a transferstep of transferring the silicon wafer from the first chuck table to asecond chuck table of a grinding apparatus;

FIGS. 7A to 7D are side views for specifically illustrating the transferstep shown in FIGS. 6A and 6B; and

FIGS. 8A and 8B are perspective views showing a grinding step ofgrinding the silicon wafer held on the second chuck table.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A specific preferred embodiment of the wafer processing method accordingto the present invention will now be described in detail with referenceto the attached drawings. Referring to FIG. 1, there is shown a siliconwafer 10 as a workpiece. The silicon wafer 10 has a front side 10 a anda back side 10 b. A protective tape 20 is attached to the front side 10a of the silicon wafer 10, so as to protect the front side 10 a(protective tape attaching step). A plurality of crossing division lines14 are formed on the front side 10 a of the silicon wafer 10 to therebydefine a plurality of separate regions where a plurality of devices 12such as ICs are formed. The silicon wafer 10 originally has a thicknessof 775 μm, for example, before grinding. The protective tape 20 iscomposed of a base sheet and an adhesive layer formed on one side of thebase sheet. The base sheet has a thickness of 100 μm, and the adhesivelayer has a thickness of approximately 5 μm. The base sheet is formed ofpolyvinyl chloride (PVC), and the adhesive layer is formed of acrylicresin.

After performing the protective tape attaching step, a holding step isperformed as shown in FIGS. 2A and 2B. Referring to FIG. 2A, there isshown a laser processing apparatus 4 (the whole configuration thereofbeing not shown). The laser processing apparatus 4 includes a firstchuck table 30 having a holding surface 32 for holding the silicon wafer10. As shown in FIG. 2A, the silicon wafer 10 with the protective tape20 attached to the front side 10 a is placed on the holding surface 32of the first chuck table 30 in the condition where the protective tape20 is in contact with the holding surface 32 and the back side 10 b as awork surface is oriented upward. The first chuck table 30 is rotatableby a rotational drive mechanism (not shown). The holding surface 32 isformed of a porous material allowing air communication, and it isconnected to suction means (not shown). In the condition shown in FIG.2B, the suction means is operated to hold the silicon wafer 10 throughthe protective tape 20 on the holding surface 32 of the first chucktable 30 under suction.

After performing the holding step, a modified layer forming step isperformed as shown in FIGS. 3A and 3B in such a manner that a laser beamis applied from laser beam applying means 40 included in the laserprocessing apparatus 4 to the silicon wafer 10 along each division line14 to thereby form a modified layer 10 c as a division start pointinside the silicon wafer 10 along each division line 14. Morespecifically, an alignment step is first performed to align a laser beamapplying position with a predetermined one of the division lines 14 byusing imaging means (not shown). Thereafter, the laser beam applyingmeans 40 of the laser processing apparatus 4 is operated to oscillate alaser beam having a transmission wavelength to the silicon wafer 10 froma laser oscillator (not shown) included in the laser beam applying means40. The laser beam oscillated is applied through focusing means 42included in the laser beam applying means 40 to the back side 10 b ofthe silicon wafer 10 held through the protective tape 20 on the firstchuck table 30 in the condition where the focal point of the laser beamis set inside the silicon wafer 10. At the same time, the first chucktable 30 is moved in the X direction shown by an arrow X in FIGS. 3A and3B at a predetermined feed speed. Accordingly, a modified layer 10 c isformed inside the silicon wafer 10 along the predetermined division line14. Although not shown, the laser processing apparatus 4 furtherincludes X moving means for moving the first chuck table 30 in the Xdirection, Y moving means for moving the first chuck table 30 in the Ydirection perpendicular to the X direction, and rotating means forrotating the first chuck table 30. By controlling the laser beamapplying means 40, the X moving means, the Y moving means, and therotating means, the laser processing operation mentioned above isrepeated along all of the other division lines 14 to thereby form aplurality of similar modified layers 10 c inside the silicon wafer 10along all of the other division lines 14.

For example, the modified layer forming step using the laser beamapplying means 40 is performed under the following processingconditions.

Wavelength: 1342 nm

Average power: 0.18 W

Repetition frequency: 80 kHz

Spot diameter: 1 μm

Work feed speed: 180 mm/second

Focal position: 70 μm from the front side 10 a (705 μm from the backside 10 b)

After performing the modified layer forming step, an unloading step isperformed as shown in FIG. 4 in such a manner that the silicon wafer 10with the protective tape 20 is unloaded from the first chuck table 30.The unloading step will now be described in more detail with referenceto FIGS. 5A to 5C. As shown in FIGS. 5A to 5C, the unloading step isperformed by using transfer means 50 (the whole configuration thereofbeing not shown). The transfer means 50 includes a transfer arm 52 and asuction pad 54 provided at the front end of the transfer arm 52 so as tobe oriented downward. The transfer arm 52 is movable horizontally andvertically by a moving mechanism (not shown). The suction pad 54 is adisk-shaped member having substantially the same size as that of thefirst chuck table 30. The suction pad 54 has a lower surface functioningas a suction holding surface 56. The suction holding surface 56 isformed of a porous material allowing air communication, and it isconnected through the transfer arm 52 to suction means (not shown).

In performing the unloading step, the transfer arm 52 is first moved bythe moving mechanism to position the suction pad 54 directly above thesilicon wafer 10 held on the first chuck table 30 as shown in FIG. 5A.Thereafter, the moving mechanism is operated to lower the transfer arm52 until the suction holding surface 56 of the suction pad 54 comes intoabutment against the back side 10 b of the silicon wafer 10 held on thefirst chuck table 30. At this time, the distance between the suctionholding surface 56 of the suction pad 54 and the back side 10 b of thesilicon wafer 10 is measured by a proximity sensor (not shown). Thus,the silicon wafer 10 is sandwiched between the first chuck table 30 andthe suction pad 54 as shown in FIG. 5B.

Thereafter, the suction means connected to the suction pad 54 isoperated to hold the back side 10 b of the silicon wafer 10 undersuction. Accordingly, the silicon wafer 10 is held under suction by boththe holding surface 32 of the first chuck table 30 and the suctionholding surface 56 of the suction pad 54. Thereafter, a suction forceapplied to the first chuck table 30 is removed, so that the siliconwafer 10 is held under suction only by the suction pad 54. Thereafter,the transfer arm 52 is lifted to thereby move the silicon wafer 10 awayfrom the first chuck table 30 as shown in FIG. 5C. Thusly, the unloadingstep is finished.

After performing the unloading step, a transfer step is performed asshown in FIGS. 6A and 6B in such a manner that the silicon wafer 10 withthe protective tape 20 is transferred to a second chuck table 60included in a grinding apparatus 6 (the whole configuration thereofbeing not shown) for grinding the back side 10 b of the silicon wafer10. In the transfer step, the silicon wafer 10 is held on the secondchuck table 60 in the condition where the protective tape 20 is incontact with the upper surface of the second chuck table 60. Thereafter,the suction pad 54 is moved away from the back side 10 b of the siliconwafer 10. As similar to the first chuck table 30, the upper surface ofthe second chuck table 60 functions as a holding surface 62 for holdingthe silicon wafer 10 under suction. The holding surface 62 is formed ofa porous material allowing air communication, and it is connected tosuction means (not shown). The grinding apparatus 6 is located adjacentto the laser processing apparatus 4, and the transfer means 50 is soarranged as to transfer the silicon wafer 10 from the laser processingapparatus 4 to the grinding apparatus 6. The transfer step will now bedescribed in more detail with reference to FIGS. 7A to 7D.

As shown in FIG. 7A, the moving mechanism for moving the transfer arm 52is operated to move the transfer arm 52 and position the suction pad 54holding the silicon wafer 10 directly above the second chuck table 60 ofthe grinding apparatus 6 for grinding the back side 10 b of the siliconwafer 10. Thereafter, the moving mechanism is operated to lower thetransfer arm 52 until the protective tape 20 attached to the front side10 a of the silicon wafer 10 held by the suction pad 54 comes intoabutment against the holding surface 62 of the second chuck table 60. Atthis time, the distance between the protective tape 20 of the siliconwafer 10 and the holding surface 62 of the second chuck table 60 ismeasured by a proximity sensor (not shown). Thus, the silicon wafer 10is mounted on the holding surface 62 of the second chuck table 60 in thecondition where the silicon wafer 10 is held by the suction pad 54 asshown in FIG. 7B (mounting step).

After performing the mounting step, a suction force applied to theholding surface 62 of the second chuck table 60 is removed, and asuction force applied to the suction pad 54 is also removed. The removalof the suction force applied to the suction pad 54 may be effected byphysically blocking a suction passage connected to the suction pad 54 orby stopping a suction pump included in the suction means connected tothe suction pad 54. Any other methods for removing the suction forceapplied from the suction pad 54 to the silicon wafer 10 may be adopted.Accordingly, the silicon wafer 10 is physically sandwiched between thesuction pad 54 and the second chuck table 60 without receiving a suctionforce from the suction pad 54 and the second chuck table 60 as shown inFIG. 7C (sandwiching step).

After performing the sandwiching step, a suction force is applied againto the holding surface 62 of the second chuck table 60, thereby holdingthe silicon wafer 10 through the protective tape 20 on the holdingsurface 62 under suction. Thereafter, the transfer arm 52 is lifted tomove the suction pad 54 away from the back side 10 b of the siliconwafer 10 as shown in FIG. 7D. Thus, the silicon wafer 10 is held by onlythe second chuck table 60 under suction (suction holding step). In thismanner, the mounting step, the sandwiching step, and the suction holdingstep are sequentially performed to thereby finish the transfer step.

After performing the transfer step, a grinding step is performed byusing the grinding apparatus 6 as shown in FIGS. 8A and 8B. Referring toFIG. 8A, the grinding apparatus 6 includes grinding means 70 forgrinding the back side 10 b of the silicon wafer 10 held on the secondchuck table 60 to thereby reduce the thickness of the silicon wafer 10.The grinding means 70 includes a spindle 72 adapted to be rotated by arotational drive mechanism (not shown), a mounter 74 fixed to the lowerend of the spindle 72, and a grinding wheel 76 mounted on the lowersurface of the mounter 74. The grinding wheel 76 is composed of a baseand a plurality of abrasive members 78 fixed to the lower surface of thebase so as to be arranged annularly along the outer circumference of thebase.

In the condition where the silicon wafer 10 is held through theprotective tape 20 on the second chuck table 60 under suction, thesecond chuck table 60 is rotated at 300 rpm, for example, in thedirection shown by an arrow 60 a in FIG. 8A, and the spindle 72 is alsorotated at 3400 rpm, for example, in the direction shown by an arrow 72a in FIG. 8A. Thereafter, the grinding means 70 is lowered to bring theabrasive members 78 of the grinding wheel 76 into contact with the backside 10 b of the silicon wafer 10. Further, the grinding means 70 is feddownward by a predetermined amount at a feed speed of 1 μm/second, forexample, in the direction perpendicular to the holding surface 62 of thesecond chuck table 60. At this time, the thickness of the silicon wafer10 may be measured by a contact type measuring gauge (not shown) duringthe grinding operation. For example, the back side 10 b of the siliconwafer 10 is ground until the thickness of the silicon wafer 10 becomes60 μm. In this preferred embodiment, the modified layers 10 c are formedinside the silicon wafer 10 along all of the division lines 14, so thatthe silicon wafer 10 can be easily broken along each division line 14.Accordingly, the silicon wafer 10 is divided into individual devicechips corresponding to the respective devices 12 along each divisionline 14 by a grinding pressure applied to the silicon wafer 10 in thegrinding operation. At the same time, the modified layers 10 c formed atthe distance of 70 μm from the front side 10 a (lower surface) of thesilicon wafer 10 are removed by this grinding operation as shown in FIG.8B. After performing the grinding step, the individual device chipsdivided from the silicon wafer 10 are transferred to an apparatus forperforming a pickup step.

With the above configuration of the present invention, the followingparticular effects can be exhibited. In the transfer step oftransferring the silicon wafer 10 to the second chuck table 60 of thegrinding apparatus 6, the silicon wafer 10 held by the suction pad 54 isbrought into abutment against the holding surface 62 of the second chucktable 60. Before applying a suction force to the second chuck table 60,the suction force applied to the suction pad 54 is removed. In thiscondition, no suction force is applied from the suction pad 54 and thesecond chuck table 60 to the silicon wafer 10, and the silicon wafer 10is physically sandwiched between the suction pad 54 and the second chucktable 60. Accordingly, it is possible to once completely relieve aninternal stress generated in the silicon wafer 10 due to the suctionholding by the suction pad 54. Further, since the silicon wafer 10 isphysically sandwiched between the suction pad 54 and the second chucktable 60, the silicon wafer 10 can be thereafter held on the secondchuck table 60 under suction without displacement. As a result, evenwhen the silicon wafer 10 is ground as receiving a grinding pressure inthe grinding step, there is no possibility of unintentional breaking dueto residual internal stress in the silicon wafer 10. Accordingly, it ispossible to prevent the problem that the devices 12 formed on thesilicon wafer 10 may be partially damaged to cause a reduction inproduction efficiency.

In the above preferred embodiment, the unloading step and the transferstep are performed by using the same suction pad 54 of the transfermeans 50 to transfer the silicon wafer 10 from the first chuck table 30of the laser processing apparatus 4 to the second chuck table 60 of thegrinding apparatus 6. However, this configuration is merely illustrativeand any other modifications may be made. For example, the silicon wafer10 may be once transferred from the first chuck table 30 of the laserprocessing apparatus 4 to another table by using the suction pad 54 inthe unloading step, and the silicon wafer 10 may be next transferredfrom this other table to the second chuck table 60 of the grindingapparatus 6 by using another suction pad in the transfer step. In thiscase, the transfer operation shown in FIGS. 7A to 7D may be similarlyperformed in transferring the silicon wafer 10 from the other table tothe second chuck table 60.

Further, while the silicon wafer 10 is used as a workpiece in thispreferred embodiment, the workpiece usable in the present invention mayinclude any wafer such that a modified layer may be formed inside thewafer along each division line and the wafer may be next thinned anddivided into individual device chips by grinding. Examples of such awafer include sapphire, silicon carbide (SiC), lithium tantalate (LT),and lithium niobate (LN) wafers.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claim and all changes and modifications as fall within theequivalence of the scope of the claim are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for dividing a waferinto a plurality of individual device chips along a plurality ofcrossing division lines formed on a front side of said wafer, the frontside of said wafer being partitioned by said division lines to define aplurality of separate regions where a plurality of devices are formed,said individual device chips corresponding to said respective devices,said wafer processing method comprising: a protective tape attachingstep of attaching a protective tape to the front side of said wafer; aholding step of holding said protective tape attached to the front sideof said wafer on a holding surface of a first chuck table under suction;a modified layer forming step of applying a laser beam having atransmission wavelength to said wafer to a back side of said wafer alongeach division line in a condition where a focal point of said laser beamis set inside said wafer after performing said holding step, therebyforming a modified layer inside said wafer along each division line; anunloading step of holding the back side of said wafer held on said firstchuck table by using a suction pad of transfer means after performingsaid modified layer forming step, and next moving said suction pad tothereby unload said wafer from said first chuck table; a transfer stepof transferring said wafer to a holding surface of a second chuck tableby operating said transfer means after performing said unloading step,next holding said protective tape attached to the front side of saidwafer on the holding surface of said second chuck table under suction,and next removing said suction pad from the back side of said wafer; anda grinding step of grinding the back side of said wafer held on saidsecond chuck table under suction, thereby thinning said wafer and alsodividing said wafer into said individual device chips; said transferstep including a mounting step of mounting said wafer held by saidsuction pad to the holding surface of said second chuck table, asandwiching step of removing a suction force applied to said suction padafter performing said mounting step, and then sandwiching said waferbetween said suction pad and the holding surface of said second chucktable, and a suction holding step of applying a suction force to theholding surface of said second chuck table after performing saidsandwiching step to thereby hold said protective tape attached to thefront side of said wafer on the holding surface of said second chucktable under suction, and next removing said suction pad from the backside of said wafer.